Cell cycle checkpoints are among the mechanisms developed by eukaryotic cells to optimally respond to DNA damage agents, such as ionizing radiation (IR) and ultraviolet radiation (UV). These controllers of cell cycle progression presumably enhance genetic stability and limit tumorigenesis in the organism. Characterization of the biochemical steps in these signaling pathways will help improve our understanding of: 1) normal cell cycle control; and 2) mechanisms of cellular transformation following DNA damage. While IR-induced cell cycle checkpoints have been extensively studied, less is known about UV-induced cell cycle arrest. Our long term goal is to elucidate the signal transduction pathways that control UV-induced cell cycle checkpoints. Our preliminary observations suggest that the ATM-RAD3 related kinase, ATR, is required for UV-induced S-phase and G2/M checkpoints. ATR is a member of the phosphatidylinositol kinase related protein family that is essential for signaling the presence of DNA damage or replication blocks and activating cell cycle checkpoints. Since most of the DNA damage-induced signaling pathways consist of kinase-dependent signaling cascades that regulate cell cycle progression, we reason that ATR plays a central role in activation of UV-induced cell cycle checkpoints through phosphorylation of its downstream targets. We propose three specific aims to dissect the role of ATR in UV-induced cycle checkpoints. In Specific Aim 1, we will investigate whether BRCA1, NBS1 and SMC1 are involved in ATR-dependent S-phase arrest, and whether ATR phosphorylates these proteins to facilitate the arrest. In Specific Aim 2, we will investigate the biochemical linkage of ATR kinase and p38 MAP kinase in UV-induced G2 arrest. In specific Aim 3, we will investigate whether RAD 17 is a DNA damage sensor for recruiting ATR to regulate the checkpoints. These studies will provide novel insights into the roles and mechanisms of the ATR kinase and, in the process, should shed light on mechanisms involved in cell cycle control after environmental DNA damage.